Conventionally, as a separator for an electrochemical element such as an electrical double layer capacitor, an electrolytic capacitor, a solid electrolytic capacitor or the like, a paper separator comprising mainly of fibers which is obtained by subjecting solvent-spun cellulose fibers or regenerated cellulose fibers under beating process (for example, see Patent Documents 1-3) has been used. Recently, with respect to the electrical double layer capacitor, increase in electrostatic capacity and voltage are advancing, and thus usage in application which requires high energy or high output, such as auxiliary power for automobile and railroad vehicle, is expected. Conventional paper separator is extremely weak in puncture strength when it is in a state impregnated with electrolytic solution. Therefore, for example, during the manufacture of a stack-up type electrical double layer capacitor, the paper separator tears or becomes punctured in a pressure applying process, thus resulting in a problematic high percentage of defective products. In addition, when exposed to high voltage of 3.0 V or higher, capacity decreases immediately since the paper separator goes under oxidation degradation, and thus the lifetime of an electrical double layer capacitor using a paper separator is disadvantageously short.
As a separator other than the paper separator, a separator for electrical double layer capacitor which is made by forming porous layer of an aromatic polyamide onto both surfaces of a non-woven fabric, the main structuring material of the fabric being fibers comprising a thermoplastic resin with a melting point of 220° C. or higher, is disclosed (for example, see Patent Document 4). Since this separator has the porous layer of an aromatic polyamide formed onto both surfaces of the non-woven fabric, there is a disadvantage that electrolytic solution permeability is poor. In addition, regarding this separator, since the area in which the aromatic polyamide porous layer, which is an insulator, comes in contact with an electrode is large, the contact resistance between the aromatic polyamide porous layer and the electrode becomes high, resulting in a problem that the internal resistance of the electrical double layer capacitor is high. Further, the manufacturing method for a separator disclosed in Patent Document 4 comprises coating both surfaces of a non-woven fabric with a polymer solution mainly containing an aromatic polyamide and an amide solvent , followed by formation of a solid film by immersing the non-woven fabric into an amide solidifying solution, and then allowing formation of a porous layer of the aromatic polyamide onto both surfaces of the non-woven fabric by immersing the non-woven fabric into a water bath. Therefore, in some cases, pores were not provided in a sufficient manner, which results in a disadvantage that the aromatic polyamide film would remain. In addition, when a polymer other than the aromatic polyamide, such as polyethersulfone or polyamide imide, is used, problems would arise since the precipitating polymer would not become porous but rather form a film, and the film would have a wrinkled form. In other words, this manufacturing method for the separator cannot be applied to polymers other than the aromatic polyamide. Here, the aromatic polyamide mentioned in Patent Document 4 is obtained by copolymerizing a wholly aromatic polyamide with an aliphatic diamine or an aliphatic dicarboxylic acid, in a ratio of 20 mol % or less with respect to the repeating unit of the wholly aromatic polyamide.
As a separator for an electrical double layer capacitor other than those described above, a separator for an electrical double layer capacitor comprising a fiber sheet containing fibers having fibril and polyester fibers having a fineness of 0.45 dtex or less is disclosed (for example, see Patent Document 5). When the thickness of this separator is decreased, especially when it is less than 40 μm, internal short circuit caused by pinhole and increase in current leakage would occur, which are regarded as problems.
As a battery separator, a combined porous film obtained by integrating a porous film A comprising a resin having a melting point of 150° C. or lower, with a porous film B comprising a resin having a glass transition temperature of 150° C. or higher, is proposed (for example, see Patent Document 6). Regarding this combined porous film, average pore diameter of the porous film A is extremely small as 0.01-0.1 μm. Therefore, in essence, the porous film B is hardly formed in the void of the porous film A, which causes a disadvantage that the porous film A and the porous film B are prone to interlayer delamination. In addition, the air permeability of the combined porous film is too low; that is, the air penetrating property is extremely poor. Therefore, when this combined porous film is used as a separator for an electrochemical element such as an electrical double layer capacitor or battery, the internal resistance of these electrochemical elements would become extremely high, which is regarded as a problem. Further, during usage, an electrolytic solution gradually leaks out from the separator, resulting in dry-up of the separator, which would cause the internal resistance of the electrochemical element to rise gradually, reducing capacity and shortening lifetime of the electrochemical element.
As a separator for a lithium ion battery, which is one type of an electrochemical element, a porous film is generally used. Since fine pores of the porous film is very small, an electrolytic solution would hardly permeate when the electrolytic solution such as an ionic liquid has a high viscosity, which is regarded as a problem. On the other hand, when the viscosity of the electrolytic solution is low, dry-up of the separator would occur, which would raise the internal resistance of the lithium ion battery and shorten the lifetime of the lithium ion battery. In addition, there was a problem that the high-rate property is poor, and that the capacity decreases rapidly when electrical discharge is conducted with high current.    Patent Document 1: Japanese Laid-open Patent [Kokai] Publication No. Hei 5-267103    Patent Document 2: Japanese Laid-open Patent [Kokai] Publication No. Hei 11-168033    Patent Document 3: Japanese Laid-open Patent [Kokai] Publication No. 2000-3834    Patent Document 4: Japanese Laid-open Patent [Kokai] Publication No. 2006-100512    Patent Document 5: Japanese Laid-open Patent [Kokai] Publication No. 2001-244150    Patent Document 6: Japanese Laid-open Patent [Kokai] Publication No. 2007-125821